1. Field of the Disclosure
The disclosure relates to a light emitting diode (LED) driving circuit. Particularly, the disclosure relates to an LED driving circuit using a single controller to drive a plurality of converters.
2. Description of Related Art
Referring to FIG. 1, FIG. 1 is a circuit schematic diagram of a conventional light emitting diode (LED) driving circuit. The LED driving circuit includes a controller 100, a first boost converter 160a, a second boost converter 160b, a common output capacitor C, a first LED string 150a, a second LED string 150b, a lowest voltage selecting circuit 140 and a current balance circuit 145. The first boost converter 160a is a direct current (DC) to DC boost converter, which includes an inductor La, a switch SWa and a rectifier device Da. One end of the inductor La is coupled to a DC input voltage Vin, and another end thereof is coupled to one end of the switch SWa, and another end of the switch SWa is coupled to ground. A positive end of the rectifier device Da is coupled to a connecting point of the inductor La and the switch SWa, and a negative end thereof is coupled to the common output capacitor C. The second boost converter 160b is a DC to DC boost converter, which includes an inductor Lb, a switch SWb and a rectifier device Db, where coupling/connection relations among the inductor Lb, the switch SWb and the rectifier device Db are the same to that of the first boost converter 160a. The common output capacitor C receives electric power transmitted by the first boost converter 160a and the second boost converter 160b to generate an output voltage Vout, so as to light the first LED string 150a and the second LED string 150b. 
The current balance circuit 145 is coupled to negative ends of the first LED string 150a and the second LED string 150b to balance currents flowing through the first LED string 150a and the second LED string 150b, so as to equalize lighting effects of the first LED string 150a and the second LED string 150b. The lowest voltage selecting circuit 140 is coupled to the negative ends of the first LED string 150a and the second LED string 150b for detecting and determining a lowest voltage of the voltages of the negative ends, and accordingly outputs a detecting signal VFB. The controller 100 generates a switching signal Sc according to the detecting signal VFB so as to control switching operations of the switches SWa and SWb.
An advantage of the above circuit structure is that the single controller can be used to drive a plurality of converters to provide larger driving capability to drive more LEDs. Since output terminals of the converters are connected to each other, in case that the converters cannot provide the same power due to different electrical characteristics of devices caused by process errors, the converter providing more power can compensate the converter providing less power, so as to improve a whole efficiency of the LED driving circuit in theory.
Input terminals of the converters are coupled to a same DC input voltage Vin, and the output terminals thereof are connected to each other. Since the output terminals of the converters are connected to each other, the same output voltage Vout is output. The converters are switched in response to the same switching signal Sc. A conversion ratio is Vout/Vin=1/(1−D), where D is a duty cycle of the switching signal Sc. Under an ideal state, the input voltage Vin, the output voltage Vout and the duty cycle D are all the same, and so currents of the inductors La and Lb are the same. However, due to process errors, conducting impedances, threshold voltages and parasitic capacitances of the switches SWa and SWb are different, inductances and parasitic resistances of the inductors La and Lb are different, and forward conducting voltages of the rectifier devices Da and Db are different, and these differences may cause different conversion ratios of the converters, and in case that the output terminals of the converters are connected to output the same output voltage Vout, a current difference between the inductor La and the inductor Lb is enlarged.
The current difference between the inductor La and the inductor Lb may cause different temperature increases of the switches SWa and SWb, and the rectifier devices Da and Db due to different heat generated thereon, or may even cause magnetic saturation on one of the inductor La or the inductor Lb with the highest current to reduce the conversion efficiency due to excessively large current. Moreover, in some application environments that the temperature increases on components are limited, for example, a backlight module of a liquid crystal display (LCD). These application environments must use a better metal-oxide-semiconductor field-effect transistor (MOSFET) (with lower conducting impedance) to suppress the heat generated by the MOSFETs, so that the cost of the LED driving circuit is increased.